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Efforts to positively influence ecological processes and water quality by manipulating the fish community (biomanipulation) are of particular importance in drinking water reservoirs. One of the principle measures employed is to increase the abundance of predatory fish species as a means of reducing planktonophagous and benthophagous cyprinids. However, there is little information available on the effectiveness of different predatory fish in biomanipulation exercises. We examined the diet of the five dominant predatory species (pike Esox lucius, zander Sander lucioperca, asp Leuciscus aspius, European catfish Silurus glanis, and perch Perca fluviatilis) in five representative reservoirs in the Morava River drainage basin (Czech Republic). Fish prey made up 75% of total food intake, with undesirable small cyprinids dominant by biomass (40%). European catfish and asp were not taken as prey and showed no sign of cannibalism. On the other hand, predation on conspecific predatory species (including cannibalism) was relatively high in perch, pike and zander, thereby reducing their net benefit overall. This little-considered aspect of predatory feeding needs to be taken into consideration in future biomanipulation stocking strategies. [ABSTRACT FROM AUTHOR]

While fish communities have been studied in Czech reservoirs for decades, monitoring has increased since implementation of the EU Water Framework Directive (2000 / 60 / EC). Despite this, many smaller drinking water reservoirs have yet to be surveyed. Between May and July 2016, we undertook a pilot-study examining the fish communities of five Moravian reservoirs (Ludkovice, Bojkovice, Hubenov, Nová Říše and Landštejn). Fish were sampled using boat electrofishing along the littoral zone and Nordic gill nets in the pelagic zone. Both methods were evaluated separately and combined as CPUE and relative abundance and biomass ( %). Nineteen fish species and one hybrid were recorded, with roach Rutilus rutilus dominating overall by abundance, but carp Cyprinus carpio, pike Esox lucius and wels Silurus glanis dominating by biomass in electrofishing samples. Predatory species represented up to 60 % of biomass in the combined sample, with an F / C index ranging from 0.5 - 3, implying a high abundance of predators. Electrofishing tended to underestimate the abundance of bleak and ruffe, while gill nets underestimated the biomass of predatory species. In some reservoirs, the abundance of pike and wels was high (though it is difficult to say if these fish were stocked or the result of natural reproduction). In others (e.g. Landštejn), zander Sander lucioperca were not registered by either sampling method, despite intensive stocking. Overall, gill netting alone appears inadequate for gaining a true picture of the fish community in reservoirs and we suggest a combination of electrofishing and gill netting in future. [ABSTRACT FROM AUTHOR]